| Summary: | The South Atlantic Ocean maintains the Atlantic meridional overturning circulation by transporting warm water equatorward and cold water poleward, its variability strongly influences the Earth climate system. Close monitoring of the South Atlantic variability is important in developing accurate circulation and climate models. This dissertation takes the advantage of a legacy multichannel seismic dataset that surveyed the South Atlantic interior along an important transect of the meridional overturning circulation, providing the first fine scale observation of the central South Atlantic that is challenging to study with conventional hydrographic measurements. The method, namely seismic oceanography, is capable of observing thermohaline structures on the order of several meters in both vertical and horizontal directions. Experiencing with this dataset, we develop a seismic processing strategy that enhances the imaging quality of the water column for legacy seismic data. Furthermore, we image the central South Atlantic down to ~1600 m and find numerous mesoscale structures formed by double diffusion and thermohaline intrusion. From the seismic images, we derive a high-resolution map of turbulent diffusivity of the central South Atlantic thermocline and find enhanced mixing caused by rough topography and a storm. We also find the important role of an eddy in promoting shear instabilities and deep propagation of near-inertial energy. Our findings suggest the prevalence of mesoscale processes and heterogeneity of turbulent mixing in the South Atlantic interior, calling for more fine scale observations in the ocean interior in order to build more accurate circulation and climate models.
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